1. Field of the Invention
This invention relates to a semiconductor photo-electron-emitting device which is a photodetecting device having sensitivity to light having long wavelengths.
2. Related Background Art
In the field of applying an electric field to a semiconductor photo-electron-emitting device in order to accelerate photoelectrons generated by the excitation of incident photons, there is generally an electrode having a Schottky junction formed on the semiconductor layer, and a bias voltage supplied by the electrode applying an electric field thereto. The conventional photo-electron-emitting devices, which use semiconductors, use this electron transfer effect. An example, which does not use the electron transfer effect, is Japanese Patent Laid-Open Publication No. 234323/1990. The electron transferring semiconductor photo-electron-emitting device of this invention relates to the above described electron transfer effect. A related electron transferring photo-electron-emitting device is disclosed by, e.g., R. L. Bell U.S. Pat. No. 3,958,143. In the R. L. Bell patent, a Schottky electrode is prepared by forming an Ag thin film, by vacuum evaporation, on a III-V group compound semiconductor. A bias voltage is supplied from the electrode to apply an electric field to the semiconductor layer so that photoelectrons are accelerated.
Such electron transferring photo-electron-emitting devices have structures as exemplified below. Incident photons h.nu. are absorbed to generate photoelectrons by excitation. An ohmic electrode is formed on one side of a semiconductor layer. On the other side thereof a Schottky electrode, being formed of an Ag thin film in the shape of an island, is formed and a Cs.sub.2 O layer is formed on the Schottky electrode. A bias voltage is applied between the Schottky electrode and the ohmic electrode in order to apply an electric field to the semiconductor layer. The photoelectrons generated in the semiconductor layer by the excitation are, thus, accelerated. The accelerated photoelectrons are transferred from a .GAMMA.-valley of the conduction band to a higher energy L-valley by an electron transfer effect (the so-called "Gun effect") before they arrive at the emitting surface where they are emitted into a vacuum.
But, in a photoelectronic conversion device having the above described photoelectron emitting surface, especially a reflecting photo-electron-emitting device, which admits incident photons on the side of the emitting surface, the incident photons h.nu. are absorbed by the Schottky electrode, formed on the emitting surface, without arriving at the semiconductor layer. This results in much deterioration of the photoelectronic conversion efficiency. In view of this, in a conventional electron transferring semiconductor photo-electron-emitting device, the Schottky electrode is formed on an about 100 .ANG. thickness thin film in order to cause incident photons h.nu. to be efficiently absorbed. It is known that when metal is evaporated on a semiconductor layer in a thickness of about 100 .ANG., the metal is distributed not in a layer, but in shapes of islands. In the above described electron transferred semiconductor photo-electron-emitting device, the Schottky electrode is in the form of islands.
Photoelectrons are generated by the excitation created when incident photons h.nu. pass through the island-shaped electrode or between islands of the electrode and are emitted into a vacuum through the Cs.sub.2 O layer. Thus, an emission probability of the photoelectron depends on a film thickness of the Schottky electrode and the gaps between the islands of the electrodes. Their control is very difficult. Furthermore, gaps between the islands of the electrodes depend on the heat treatment following the evaporation. Degassing and cleaning at high temperatures are impossible. Eventually the electrode's performance as the photo-electron-emitting surface deteriorates greatly.
Thus, the Schottky electrode film thickness and the gaps between the islands of the electrode greatly influence the optical transmission of incident photons h.nu., and an emission probability of photoelectrons into the vacuum, which are generated by the excitation of the incident photons h.nu.. It is difficult to fabricate a stable Schottky electrode with high reproductivity. Thus, the conventional electron transferring semiconductor photo-electron-emitting devices have not been put to practical uses.
An object of this invention is to provide an electron transferring semiconductor photo-electron-emitting device that includes a stable, heat-resistant Schottky electrode formed with a high reproducibility rate. A further object of the present invention is to provide an electron transferring semiconductor photo-electron-emitting device that has an improved transmission of incident photons and emission probability of the photons into a vacuum, whereby photodetection having a high sensitivity can be realized.